numerical modeling of 3-d turbulent two-phase flow and coal combustion in a pulverized-coal...

ACTA MECHANICA SINICA (English Series), Vol.13, No.3, August 1997 The Chinese Society of Theoretical and Applied Mechanics Chinese Journal of Mechanics Press, Beijing, China Allerton Press, INC., New York, U.S.A.

ISSN 0567-7718

N U M E R I C A L M O D E L I N G OF 3-D T U R B U L E N T T W O - P H A S E FLOW A N D COAL C O M B U S T I O N IN A

PULVERIZED-COAL COMBUSTOR*

Zhou Biao** ( ~ ~ ) Wu Chengkang ( ~ )

(Combustion Laboratory, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, China)

A B S T R A C T : In the present paper, a multifluid model of two-phase flows with pulverized-coal combustion, based on a continuum-trajectory model with reacting particle phase, is developed and employed to simulate the 3-D turbulent two-phase flows and combustion in a new type of pulverized-coal combustor with one primary-air jet placed along the wall of the combustor. The results show that: (1) this continuum- trajectory model with reacting particle phase can be used in practical engineering to qualitatively predict the flame stability, concentrations of gas species, possibilities of slag formation and soot deposition, etc.; (2) large recirculation zones can be created in the combustor, which is favorable to the ignition and flame stabilization.

K E Y WORDS: numerical simulation, pulverized-coal combustor, two-phase flow

1 I N T R O D U C T I O N

It is well known that the particle t rajectory model[ 1'2] has been most widely used in

modeling pulverized-coal combustion. With this model, the particle history effect can be

easily taken into account and false diffusion can be avoided in modeling particle phase, but it is difficult for this model to fully consider the turbulent t ransport of particle mass, par-

ticle momentum and particle energy, and is difficult to give detailed information of particle velocity and concentration comparable with experimental results. In order to compare the predictions with the experimental results, it is necessary to calculate a large number of tra-

jectories. At the same time, the pure t rajectory model suffers from the difficulty in dealing with particle entering or leaving the recirculating zone. The continuum model can fully account for the particle turbulent transport , and can give more detailed information which can be compared with experimental results, but it is difficult to consider the particle his-

tory effect. The continuum-trajectory model with reacting particle phase, proposed by Prof. L. X. Zhou [3,4], overcomes these disadvantages and retains the advantages of both models mentioned above. This model has been successfully used in modeling 2-D pulverized-coal combustion[5].

Received 6 January 1997 * Sponsored by the National Key Projects of Fundamental Research of China. ** Present Address: Department of Mechanical and Aerospace Engineering~ Princeton University,

Princeton, NJ 08544, USA

194 ACTA MECHANICA SINICA (English Series) 1997

A new type of pulverized-coal combustor possibly having multiple functions and low pollutant emission has been proposed [6]. The chief aims are to increase the residence time of coal particles and to solve the problem of slag formation in the combustor. Figure 1 shows the schematic diagram of the combustor. A pair of primary air jets and a pair of control jets enter the combustor along the wall, as shown in the figure. The air jets along the wall can effectively protect it against overheating and erosion and can effectively solve the problem of slag formation. The flowfield characteristics may be utilized to achieve a multiple of functions, which include boiler startup, low load flame stabilization and main burner operation. As the first step of research, 3-D numerical simulation of two-phase flow in this combustor has been carried out. Some useful guidelines have been obtained through this work.

1 / / -

/ /

\ \

Fig.1 Schematic diagram of the proposed combustor 1,3--primary air; 2,4~controlling air

In the present paper, in order to develop the computer code of this multifluid model of two-phase flows with pulverized-coal combustion, the calculation conditions are simplified. In the simplified case, the controlling jets (air jets No.2 and 4) and one primary-air jet (air jet No.3 ) are turned off and the remaining primary-air (air jet No.1 ) is turn on. A 180 ~ sector is taken as the computed domain in the r-0 plane. The primary-air inlet is located in the x-r plane of 0 = 0 ~ Some important results are obtained.

2 N U M E R I C A L M O D E L S A N D S O L U T I O N P R O C E D U R E

Based on the continuum concept of multiphase flows and the k-e-Ap two-phase turbulence model[7], the gas phase governing equations for 3-D turbulent reacting gas-particle flows in cylindrical coordinates can be written in the following generalized form

1 0 rpvr 1 0 0 , v 0r 1 O , _ 0r ~ 1 6 2 ; ~ ( ) + ; ~ ( ~ r = ~ , ~ j + ~ r l ~ +

1 a ar r2 ~ ( P r ~- Sr + SCp (r = 1 , u , v , w , k , e , h , Y , ) (1)

The meanings of r Fr and Sr are the same as those in modeling of pure gas phase, and Scp

is--~"~nkink, - - 2 ( u - - u k ) - - u ~ n k f n k , --~_, Pk ( v - - vk ) - - v~_ ,nkrnk , - - ~ . l r k ~-r~

wk) -- w ~ nkfnk , 0 , 0 , ~ nk/nsk , ~ nkQk -- h ~_, nkrhk , --as ~ nkmk , in different equations, and the subscript s denotes CHr CO, CO2, 02 and H20.

Vo1.13, No.3 Zhou & Wu: 3-D Turbulent Two-Phase Flows and Coal Combustion 195

The time-averaged reaction rate is determined by the EBU-Arrhenius model:

W, = min(W,.EBU, Ws.Ar )

w , Esv = C n p Y ~ u e / k

Ws.ar = Bsp2ysYOx e x p ( - E / R T ) (s = CH4, CO)

where W is reaction rate, Y is mass fraction, subscript Ar denotes Arrhenius Law, EBU denotes eddy-break-up model, FU denotes fuel.

The time-averaged particle-phase continuity and momentum equations for 3-D turbulent reacting gas-particle flows in cylindrical coordinates can be written in the following generalized form

_ _ 0 0 0 (pkukr + ; - ~ (rpkVkr + --~(Pkw~r = Ox

~ + ; - b T : ~ + - ~ - ) + . = = == s+~ + s~k~ (2)

where Ck, F+, S+k and S+kg denote the generalized variable, transport coefficient and source term of particle phase respectively, and their meanings for each equation are shown in Tab. 1.

The equations of mass and temperature change of a pulverized-coal particle, using the two=equation model of devolatilization and three-reaction model of diffusion-kinetic char combustion, can be written as follows[ r]

~nk~ = rdkguDpln[1 + (Y~, - Y ~ ) / ( 1 - Y~,~,)]

Yw~ = B~ exp(-E~,/RTk)

r = mckaBv exp( - E , / RTk )

dmck dt -- --m~kB, exp ( -E , /RTk )

~hh~ = ~[TrdkY~, ~ S l exp(-E1/RTk)]

~rd~ C dTk ~rd2ae(T 4 - T~) - r + r

where subscript w denotes wall, g denotes gas, v denotes volatile, c denotes raw coal, and p denotes particle.

The particle mass and temperature change due to reactions is traced along the trajec- tories obtained from the Eulerian predictions, by using these ordinary differential equations and algebraic expressions. These ordinary differential equations are solved by using 4th order Runge-Kutta method Is].

The particle turbulence is modeled by using the particle-tracking-fluid (Ap) model

v~k = (1 + r ,k)_l /]T TT

where

"r~k = d2pp- 1.5C~k/e 18# T T ~-


Table 1 Equations for particle phase

Equations r a ~ I~s SCS Scaa

vs Particle continuity 1 Ps Pa' ~ narha 0 ~p

Oua 0 "r Ova ,+ pa (u - ua)-I- Particle ua Pa 1 Ps (P~ 'Oz ) + "~r ( Pa-~z ) wrs

0 �9 O w s . + O , uaOps . axial momentum ~-~ (/~a--~--x) ~ [uk ~--p -~-~x-x )+

0 (rua ua • r o t ap --~-" "4- Psg= +

o , uaOos , o ( ~ , a u s O p a ) + ap Oz

0 , uaOpa. 0 . usOpa,

0 Oua 0 . Ova. 1 2 Ps Particle va Pa 1 Pa -~x(Pk'-~r )+-~-O-r(rPk-~r )+- -psWS+ - - ( v - v a ) +

r. ~rh

0 " .O'Wa ?)]_2 tow a ~-)-F vnarha radial momentum ~-~ [US ( ~ r/~S k"~'~ +

0 "v ~'a Ops, + Opa _ ~ s~-g;;j { ( , ~ s uA - ~ - )+ ~p

r ~ O (vs ua OPa ~ + O , us Opa , ,

O v a O p s ) + 0 . u sOpa)_

2 us Opa

~ a ~ o-T + pag.

una Cnk

O auk O r ,a rk wk)]_b Pk ( w - - ~ k ) + Particle wk Pk 1 /~k ~-~x(Pkr--~) + ~ r t r / ~ k ~ - ~ r 1"r k

0 , p k . Owk tangential momentum Pkgo + ~-~ I--r-- [ ~ + 2vk )]-- wnarha

0wk wu r pkvswk + r "cO0 Or r

0 uhOpa)+ 0 uaOpa)+

wk uk Ops va ua Opk + - r ap Or r 2 ap

Both gas-phase and particle-phase conservation equations in the Eulerian coordinates are integrated in the computational cell to obtain the finite difference equations by using the hybrid scheme for the convection terms, and the linearization of the source terms. The finite difference equations are solved by TDMA line-by-line and plane-by-plane iterations with under-relaxation[9]. Multiple iterations between the two phases have been adopted.

Some additional formulas are shown in Table 2. The grid system contains 33 • 18 • 14 -- 8 316 nodes in the computational domain (r mm • 1200mm). The initial diameter of coal particles is 60 ~m. The computer code has been developed with nearly 20000 FORTRAN77

Vol.13, No.3 Zhou & Wu: 3-D Turbulent Two-Phase Flows and Coal Combustion

Table 2 Addit ional formulas

G k -~ p T ( 2 [ ( 0 ~ ) 2 + ( ~ ) 2 + ( 1 0 w

0 x + V ~ ) + 0 T + 0 x +(~_g~+~_;_~)2

197

I.te = p T d- p

I.tk -~ PkVk

.r T _~ C T k / e

I~T -~ C u p k 2 / e VT -~ DT / P

T ! Vk / I /T -~ (1 -t- "rk )--1 "r' ~k = pd~/18"

TT

ppd~ (1 + 0.15Re~ -1 exp(Bk) -- 1 Tr k -~ " ~ p B k

statements, and running one case in a microcomputer 486-66 spends about 170 CPU hours.

3 R E S U L T S A N D D I S C U S S I O N S

3.1 Stability of Pulverized-coal Flame

(1) Configuration of recirculation zones

Figure 2 shows that there are obvious gas and particle recirculation zones, both in 8 = 0 ~ 180 ~ plane and 8 = 90~ ~ plane.

(2) Figure 3 is the gas-phase temperature contour map in coal combustion. It can be seen that the high temperature zones of gas phase are located in the gas and particle recirculation zones.

(3) Figure 4 gives the coal particle concentration contours in coal combustion. It can be seen that the particle concentration in the recirculation zones (especially in x - r plane of 0 = 90 ~ 270 ~ is high.

In summary, Fig.2 through Fig.4 illustrate that , in the combustor, there are several strongly recirculating zones, in which the particle concentration and the gas-phase temperature are high. Such features can be used to stabilize the coal flame.

3.2 Prediction of Slag Formation and Soot Deposi t ion

The reasons of slag formation mainly include the following items: melting of coal particles due to their high temperature; high temperature of the wall of the combustor; impingement against the wall by the coal particles. The reason of soot deposition is mainly that, in the combustor, there exist dead zones in which the velocity of gas/particle is nearly zero. The melting point of the coal used in this research is about 1573 K. From Fig.3, it can be seen that there are possibilities of slag formation and soot deposit in the combustor with only one-primary-air jet. From the velocity vectors in the r-8 planes (Fig.5), such places can be predicted. In the places where the particles impinge against the wall directly, slag may be formed; in the places where the particle/gas velocity is nearly zero, soot may be deposited.


Fig.2 Velocity vectors

3.3 P r e d i c t i o n o f C o n c e n t r a t i o n for G a s Spec ies

Figure 6 and Fig.7(a) are the concentration contour maps for volatile and oxygen re-

spectively. The volatile concentration in the reverse-flow zones is nearly zero except the

reverse-flow zone near the outlet at the x - r plane of 0 = 90 ~ 270 ~ where the volatile con-

centration is higher due to the reverse flow of the gases at the plane of 0 = 0 ~ which contain

higher concentration of unburnt volatile. The oxygen concentration in the reverse-flow zone

is lower than that in other zones. Such features illustrate that these recirculation zones are

very important to devolatilization, ignition, and stable combustion.

Vol.13, No.3 Zhou & Wu: 3-D Turbulent Two-Phase Flows and Coal Combustion 199

Fig.3 Temperature contours

Fig.4 Particle concentration contours


Fig.5 Velocity vectors in the r-O planes of two-phase combustion (x--690 mm)

Fig.6 CH4 concentration contours

Figure 7(b) and Fig.7(c) are the contour maps for the concentration of carbon dioxide and water vapour respectively. These concentrations in the recirculation zones are higher,

which illustrates the high- temperature gases can be entrained by the recirculating flow. Such

feature is b~eneficial to the stability of coal flame.

Figure 8 is the contour map for carbon monoxide concentration in the combustor. In

the zone with axial distance x less than 0.8 m, the carbon monoxide concentration is nearly

Vol.13, No.3 Zhou & Wu: 3-D Turbulent Two-Phase Flows and Coal Combustion 201

1.20

1.10

1.00

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0.00 -0.15 0.00 0.15

(a)02 (0=0 ~ 180~

1.20

1.10

1.00

0,90

0.80

0.70

0.60

0,50

0,40

0.30

0.20

0,10

. . . . 0.00 -0.15 0.00 0.15 -0.15 0.00 0.15

(b)CO~ (O=0 ~ 180 ~ (c )H20 (0=0 ~ 180 ~

Fig.7 Concent ra t ion contours

! .20

1.10

1.00

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

O.OC -0.15

I

0

0.00 0.15

1.20

1.10

1.00

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0.00 -0.15 0.00 0.15

(a) two-phase combust ion (b) two-phase combust ion

(0=0 ~ 180") (0=90 ~ 270 ~

Fig.8 CO concent ra t ion contours


zero, while in other zones the carbon monoxide concentration is higher. This exactly illus-

trates the recirculation zone near the outlet is very important to char combustion.

4 C O N C L U S I O N S

(1) The computer code for the numerical modeling of 3-D turbulent two-phase flows and pulverized-coal combustion based on a continuum-trajectory model with reacting particle

phase has been successfully developed and has been successfully applied to the modeling

of a combustor with one primary-air inlet along its wall.

(2) The prediction can qualitatively give very useful information in practical engineering, such as, the stability of flame, possibilities of slag formation and soot deposit, concen-

trations of gas species, etc.; So this prediction method as an optimizing tool can be used to model the practical two-phase flows with pulverized-coal combustion in combustors

or furnaces. (3) The prediction results show that the gas and particle recirculation zones are very impor-

tant to devolatilization, volatile ignition and flame stabilization, especially, the recirculation zone near the outlet at the x - r plane of 0 = 90 ~ and 270 ~ is crucially important

to char combustion.

A c k n o w l e d g e m e n t This work has been supported by the National Key Projects of Fundamental Research of China. We acknowledge the contribution of many other members,

Professors Lifang Chen, Huanqing Zhan, and Wenchao Sun, of the Combustion Laboratory, Institute of Mechanics, Chinese Academy of Sciences.

R E F E R E N C E S

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3 Zhou LX, Zhang J. A Langrangian-Eulerian particle model for turbulent two-phase flows with reacting particle. In: Zhuang Fenggan ed. Proc 10th Inter Conf on Num Meth in Fluid Dyn. Berlin: Springer-Verlag, 1986. 705N709

4 Zhou LX. A multi-fluid model of two-phase flows with pulverized-coal combustion. In: Feng Junkai ed. Coal Combustion. N.Y.: Hemisphere, 1988. 207~,,213

5 Deng Zuobo, Zhou Lixing. Numerical modeling of the combustion of pulverized-coal in a con- trolled mixing-history furnace. Proc of 2nd Inter Syrup on Coal Comb. Beijing: China Machine Press, 1991. 114,,~121

6 Zhou B, Wu Chengkang. 3-D numerical simulation of two-phase flows in a proposed pulverized- coal combustor with multiple functions and low emission of pollutants. In: Leung TP ed. Proc 3rd Asian-Pacific International Symposium on Combustion and Energy Utilization, Dec.llN15,1995, Hong Kong, Hong Kong Polytechnic University, 216

7 Zhou Lixing. Theory and Numerical Modeling of Turbulent Gas-Particle Flows and Combustion. Beijing: Science Press and Florida: CRC Press, 1993

_8 Donald Greenspan, Vincenzo Casulli. Numerical Analysis for Applied Mathematics, Science, and Engineering, Addison-Wesley Publishing Company, Inc, 1988. 112,,~117

9 Patanker SV. Numerical Heat Transfer and Fluid Flow. Hemisphere, 1980

numerical modeling of 3-d turbulent two-phase flow and coal combustion in a pulverized-coal...

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